1. Field of the Invention
The present invention is broadly concerned with food delivery systems designed to maintain food at a selected temperature over relatively long periods of time. More particularly, the invention pertains to such food delivery systems which include a magnetically heatable thermal storage device within a food-holding container, wherein the storage device may be selectively heated within said container by an induction charging station. In preferred forms, the charging station indefinitely maintains the selectively heated portion of the thermal storage device at a user-selected regulation temperature by using contact-less feedback from said device.
2. Description of Prior Art
The problems associated with the delivery of hot foods to consumers has in recent years taken on greater significance owing to the growth in convenience foods and those delivered directly to households. Although the rise in pizza deliveries is a prime example, other foods are now commonly delivered to the door, from simple hot sandwiches to complete meals.
For instance, most prior art pizza delivery systems consist simply of a partially insulated, non-sealing vinyl bag or sometimes a well-insulated nylon bag into which one or more cardboard boxes containing pizzas are placed so as to maintain the pizzas as warm as possible during delivery to the customer. Although the sauce layer of a freshly cooked pizza is typically over 200 F., the sauce layer upon delivery is often as low as 110 F., particularly where delivery times in excess of 30 minutes are experienced.
The problem of cold-delivered pizzas is only partly due to inefficient delivery bags and the like. In a typical pizza operation, once a pizza emerges from the oven it is removed and placed upon a cutting table to be sliced. The pizza is then placed in a cardboard box. Very commonly, two or more pizzas are to be delivered to the same address and multiple pizza bags full of pizzas are delivered to several different customers on the same delivery run. Under these circumstances, the boxed pizzas are placed under infrared heating lamps until all pizzas for a given run have been prepared, sliced and boxed. Due to the logistics involved in such operations, some pizzas can be almost cold before the delivery run even commences.
In 1998, Dominos Pizza introduced the Heat Wave(trademark) pizza delivery system. This consists of an insulated nylon pizza bag, a wax-filled resistively heated plastic-coated thermal storage disk, and a rack charging system into which up to 20 thermal storage disks can be plugged so as to charge them with thermal energy. This system has several drawbacks. The thermal storage disks are heavy, weighing in excess of three pounds. Thus, the delivery container is no longer lightweight once the disk is in place. Furthermore, the disk requires a substantial time to become fully charged with thermal energy, taking over two hours from room temperature and over thirty minutes after a typical delivery to be fully charged. Additionally, the thermal storage disks must be plugged into and out of the charging rack, thus requiring the operator to perform additional steps. Finally, to implement the rack charging system, a typical pizza parlor must be substantially modified in terms of its power supply network and floor space to accommodate the rack.
There is accordingly a need in the art for an improved food storage and delivery system which will permit the purveyor to maintain the food products at or near a desired temperature over sustained periods, while also allowing delivery under conditions to substantially maintain this temperature. An effective hot food storage and delivery system thus requires a lightweight delivery container, a fast-charging thermal storage device capable of storing and efficiently releasing large amounts of thermal energy, and easy to operate equipment not requiring skilled labor.
The present invention overcomes the problems outlined above and provides a food delivery system broadly including a food delivery container equipped with a thermal storage device with the latter being heated while in the container by a magnetic induction charging station. Thus in the case of a pizza system, a flexible insulated bag or hard-sided container is equipped with a thermal storage device designed to remain within the bag throughout its operation. This thermal storage device includes a heat pellet; the pellet has a ferromagnetic heating element which preferably is surrounded by synthetic resin heat retentive material. In order to charge the bag or container, it is simply placed upon a charging station including a magnetic induction coil having temperature maintenance control circuitry that requires no connection to the bag or container; this serves to quickly heat the heat retentive pellet and to maintain it at a user-selected temperature without overheating. When a food item is prepared, it is placed within the bag or container for delivery. Temperature maintenance during delivery is assured because of the very significant thermal energy stored in the heat retentive pellet.
The preferred system of the invention employs a magnetic induction charging station, having a magnetic induction cooktop which is capable of infusing a vast amount of thermal energy into coupled heat retentive pellets in a very short amount of time. For instance, for pizza applications, it has been found that approximately 150,000 joules of thermal energy must be added to a room temperature pellet, and that the pellet should be brought to a surface temperature of around 230 F. in less than about 4 minutes. The charging stations and heat retentive pellets of the invention can readily meet these demanding standards. Furthermore, the preferred charging station is capable of maintaining the pellet temperature indefinitely without any cords or other leads connecting the charging station and heating element, regardless of variations in thickness of the associated containers or other specific conditions of the containers. Finally, the charging stations of the invention are capable of charging a given heating element to the predetermined regulation temperature notwithstanding the initial temperature of the element, which will be variable over the course of several delivery runs and returns to the food preparation location.
The thermal storage devices of the invention are lightweight and ruggedly constructed so as to endure heating/cooling cycles. The pellets are able to withstand very fast charges and can release approximately 75,000 joules of energy during a 30 minute delivery cycle to the container contents for temperature maintenance. A particular advantage of the thermal storage devices is that they are sized to fit within standard pizza bags without modification thereof.
As indicated, the systems and methods of the invention utilize magnetic induction as an energy transfer means in order to charge heat retentive pellets coupled in a magnetic field. Moreover, the invention employs the concept of interrupting the continuous production of a magnetic field at user-selected regulation temperatures in order to heat the heating elements to a temperature and to maintain that temperature over time. To this end, various types of feedback parameters related to the impedance of the load presented to the magnetic induction cooktop by the heating element may be used to determine whether and when to interrupt the cooktop""s magnetic field.
For example, the feedback parameter may be the amplitude of the resonant current flowing through the work coil of the induction cooktop, or alternately the absolute value of the rate of change of the resonant current amplitude over time. Most preferably however, periodic amplitude measurements of the current flowing through the work coil are taken and this raw data is used by the cooktop""s microprocessor to periodically compute the absolute value of the rate of change of the resonant current amplitude. The microprocessor employs an algorithm that uses both the absolute value of the rate of change of resonant current amplitude and the exact value of resonant current amplitude to determine whether and when to interrupt continuous production of the magnetic field.
Thus a preferred method of the invention involves heating a ferromagnetic heating element by magnetically coupling the element with the magnetic field of a magnetic field generator, the latter having an induction work coil and a resonant circuit that includes the work coil. The improvement of the invention comprises the steps of controlling the temperature of the element about a regulation temperature above the element""s Curie temperature by periodically determining at least two parameters of the resonant circuit related to the amplitude of the resonant current passing therethrough during element heating; in response to the determining step, the field strength of the magnetic field is altered when at least one of the parameters is above or below a selected value correlated with the regulation temperature. The parameters are advantageously the amplitude of work coil current during inverter on times and the rate of change of this current amplitude.
Although the method of the invention contemplates any kind of field altering, generally the magnetic field is fully interrupted when a parameter is above or below a selected value. Furthermore, the regulation temperature is normally above the Curie temperature of the heating element and between this Curie temperature and a xe2x80x9cshelf temperaturexe2x80x9d defined herein.